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| A study of MADAGASCAR AMETHYST |
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| ( SiO2 ) Faceted Amethyst click here ! |
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| (Variety of Quartz) |
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| · Crystallographic properties: |
| Trigonal System (Subdivision of the hexagonal system) |
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| Crystal structure of amethyst : |
| Quartz forms hexagonal prisms at the blunted ends and the head |
| finishing in hexagonal pyramids. |
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| · Physical properties |
| Hardness 7 |
| Density 2.65 - 2.66 |
| Refractive Index: 1.54 -1.55 + 0.009 positive uniaxial |
| Glare: vitreous |
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| · Chemical Composition: |
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| Silicium Dioxide ( SiO2 ) |
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| The colouring of amethysts is due to the presence of colour centers which come from substitution of ions of |
| silicon by iron ions in the crystal lattice of quartz. |
| For example, the amethyst crystallizes at lower temperatures than smoky quartz.In Madagascar, we find it either in |
| crypts of pegmatites, or in quartzite veins in connection with those. |
| The geodes of the siliceous nodules of basalts contain some too. |
| The first have an hexagonal network, although their pattern of crystallization is only of ternary order. The seconds |
| have a ternary network. The elementary mesh is a rhomboedron, i.e. a parallelepiped consisted six equal |
| rhombuses. |
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| A ternary axis A3 joint the tops of the regular trihedrons, three normal binary axes A2 with the ternary axis joining |
| the meddle of opposite horizontal corners. |
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| Here elements of symmetry of the Trigonal system with |
| the oblique shape, the tetragonal scalenoedron. |
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| It is interesting to see what becomes this tetragonal scalenoedron in others classes where the disappearance of |
| the symmetry planes makes decrease of half the number of the faces. We obtain a trapezohedron (One notices that |
| these two trapezohedrons are not superposable. It is said that they are two enantiomorphism shapes. They are |
| symmetrical compared to a symmetry plane.) |
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One speaks about right trapezohedron and left trapezohedron (just as we have a right hand and a left hand,
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nonsuperposable thus enantiomorphism).
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| Certain minerals present sometimes left forms sometimes right forms. The most known example is that of quartz. |
| The distinction between left quartz and right quartz has a certain importance in the electronics industry where one |
| uses the piezo-electric properties of this mineral. |
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| The negative crystals of hexagonal shape can contain liquids like water, gas, as well as rutile needles, tiny hematite |
| discs, goetite, fushsite or mica. |
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Inclusions of H2O and CO2 in an amethyst from Madagascar. Habitus type of flattened Dauphiné. |
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This marvellous Bi-terminated amethyst of 128 grams also contains water drops in its center. Habitus type of rhomboedron.
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The amethyst scepters are also typical in Madagascar. |
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Here, a scepter in the blackberry shape. |
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There, the stem of the scepter developed by crystallizing around it a quantity of small points in hedgehog. Length:40 mm.
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| Amethyst in castle on its quartz gangue. 986 grams.
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| The fact that sometimes a thin surface layer of violet color be only presents in the stone or that this color is not |
| homogeneous, makes often a difficult cutting. The art of lapidary is to place the color correctly in order to make |
| homogeneous the tone of cut stones. The distribution of the color in a cut stone is observed simply by |
| immersing it in water contained in a transparent container and colourless. The color of amethyst varies from purple |
| reddish to purple bluish while passing by straightforwardly crimsons tone. Thus of very variable intensity, the color |
| is often laid out in stripes parallel to the final faces of the crystal. |
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Your guide to GGGems |
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©  All of the pictures on this website have been shot by gggems.com |
Alain Darbellay |
| Text written by Alain Darbellay. |
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